Sulfo-NHS-Biotinis a water soluble biotinylation reagent used for simple and efficient labeling of antibodies, proteins and primary amine-containing molecules. Biotin is a vitamin which binds with high affinity to avidin and streptavidin proteins. Biotin is small (244 Da), and may be conjugated to proteins without altering their biological activities. NHS-activated biotins couple to primary amino groups (-NH2) in pH 7-9 buffers to form stable amide bonds. Sulpho NHS biotin has also been used for labeling of membrane proteins; due to the charge of the sodium sulfoxide group on the succinimidyl ring, it cannot permeate the cell membrane. If the cell remains intact, only primary amines exposed on the surface will be biotinylated.